This invention relates to a method of controlling the operation of the bowl mills in a coal-fired steam generating power plant and in particular to a method of controlling the operation of such bowl mills such that the furnace of a steam generating power plant can more rapidly respond to abrupt changes in the demands placed upon the output of the furnace due to the abrupt changes in the power requirements of an electric power grid.
It has long been known in the prior art to provide apparatus for purposes of effecting the grinding or pulverizing of certain materials. More specifically, the prior art is replete with examples of various types of apparatus that have been used to effect such grinding of a multiplicity of materials. Coal is one such material wherein there is a need that it be ground to a particular fineness in order to render it suitable for the use in, for example, a fossil fuel(i.e., coal)-fired steam generating power plant.
For purposes of the discussion that follows, the coal-fired steam generating power plant referred to above is considered to consist of essentially the following major operating components: a coal feeder, apparatus for pulverizing the coal, a distribution system for distributing the pulverized coal, a furnace to which the pulverized coal is to be distributed and in which in it is to be burned and the requisite controls to effect the proper operation of the coal-fired steam generating power plant. Of particular interest herein is that portion of the steam generating power plant known as the coal pulverizer. Coal pulverizers are not new. They have been known to exist in the prior art for more than half a century. Furthermore, many improvements in the construction and/or mode of operation of coal pulverizing have been made during this period.
One particular type of coal pulverizing apparatus which is to be found in the prior art is that which is most commonly known in the industry as a bowl mill. The bowl mill gets its name from the fact that the pulverization, i.e., the grinding of the coal to a particular fineness, is accomplished on a grinding surface that bears resemblance to a bowl. Reference may be had, by way of exemplification and as a means of teaching the nature of the construction and the mode of operation of a prior art bowl mill suitable for use in a coal-fired steam generating power plant, to U.S. Pat. No. 3,465,971 which has been assigned to the same assignee as the present application. As taught by the aforementioned patent, a bowl mill consists of essentially the following major operating components: a separator body in which the bowl shaped grinding surface, i.e., a grinding table is mounted for rotation, a plurality of grinding rollers that cooperate interactively with the grinding table to effect the grinding of coal interposed therebetween, a coal supply means for supplying to the interior of the bowl mill the raw, untrammeled coal that is to be pulverized and an air supply means for supplying, also to the interior of the bowl mill, the air required for entrainment of pulverized coal of a certain fineness.
In accordance with the mode of operation of such a bowl mill, the coal, which is to be ground to a particular fineness, is introduced into the central portion of the bowl mill from above. A first pulverizing of the coal is accomplished by virtue of the cooperative interaction of the grinding table and a plurality of grinding rollers. The grinding table is made to rotate about a vertical axis, central to the separator body, while the grinding rollers are each freely rotatable about its own axis. The grinding rollers are suspended within the separator body so as to exert pressure, either by mechanical means or due to their own massive weight, against the grinding table and the coal trapped therebetween, thus effecting the pulverization of the coal. The grinding rollers are made to rotate by the mutual contact of the coal being pulverized with the grinding rollers and the rotating grinding table. The periphery of the grinding table is spaced from the interior of the walls of the separator body so as to provide an annular passage therebetween. Pressurized air, commonly known as primary air, is admitted to the lower portion of the separator body from beneath the grinding table so as to create an upwardly mobile stream of air flowing through and about the annular passage by way of a multiplicity of annular spaces formed between the periphery of the grinding table and the inner wall of the separator body. The annular spaces cause the primary air stream to flow over the grinding table. After the first pulverizing action, the coal particles are thrown outwardly from the grinding table by the effect of noninertial (i.e., centrifugal) forces. This initially ground coal contains a range of very coarse to very fine coal particles and is entrained by the primary air stream after the primary air stream passes through the annular spaces. In addition, the primary air stream must maintain a minimum velocity in order to adequately entrain coal particles of a certain fineness. Thus, there is a first stage separation of the coarsest (and therefore heaviest) coal particles from the primary air stream. These coal particles are immediately returned to the grinding table to undergo a second pulverizing action. The primary air stream, containing still relatively coarse and relatively fine coal particles, continues to flow upwardly within the separator body, thence through a convoluted path that acts to further separate still relatively coarse (and therefore still heavier) coal particles from the primary air stream in a second stage. These particles are also returned to the grinding table to suffer a second pulverizing action. However, coal particles of a particular fineness remain entrained within the primary air stream and are carried through the remainder of the bowl mill. These particles finally exit the bowl mill and are delivered to the furnace of the steam generating power plant for combustion therein. The action of a first grinding of the coal, followed by initial entrainment of some coal particles within the primary air stream, followed by the first and second stage separation of the coarser coal particles from the primary air stream, followed by the return of those coarser coal particles for a second grinding is a cycle that may be repeated several times before a particle of coal is reduced to sufficient fineness so as to be carried through the bowl mill and delivered to the furnace. As a consequence of this cyclic action there is a buildup of coal particles within the bowl mill during its steady state operation. Such buildup of coal particles is herein referred to as a slug and will be addressed in more detail below.
Of more particular interest is the means by which the aforesaid second stage separation of the more coarsely ground coal particles from the primary air stream is effected. The means by which this separation is generally accomplished is by way of a static classifier or a rotary classifier. In a static classifier the flow of primary air combined with those coal particles still entrained therein is directed through a series of stationary turning vanes which make up the aforesaid convoluted path. Said turning vanes are canted at an angle to the direction of the flow of the stream of primary air and coal particles so as to cause the coarsest (and therefore heaviest particles) to fall out of the primary air stream and return to the grinding table to suffer a second pulverizing action. In a rotary classifier the flow of primary air combined with those coal particles still entrained therein is directed through a series of vanes disposed as an inverted, truncated cone and revolving about the central vertical axis of the housing at a predetermined rotational velocity in a squirrel cage fashion. The vanes are canted at an angle to the direction of the flow of the stream of primary air and coal particles entrained therein so as to present to the stream a window through which the stream of primary air and coal particles may pass unimpeded. However, the rotational velocity of the vanes coupled with the velocity of the primary air stream and the coal particles entrained therein acts to separate the coal particles into two groups. A first group of particles are those that are relatively coarse or heavy and therefore unable to pass unimpeded through the aforesaid window and are thus returned to the grinding table to suffer a second pulverizing action. A second group of particles are those that are relatively fine or light and therefore able to pass unimpeded through the window and thus be directed through the remainder of the bowl mill and delivered to the furnace of the steam generator. For a fixed velocity of the primary air stream, by the judicious manipulation and control of the aforesaid rotational velocity of the vanes, the relative fineness of the two groups of coal particles may be adjusted, i.e., by increasing the rotational velocity of the vanes, the fineness of the coal particles that pass through the aforesaid window increases. In other words only increasingly finer particles will pass unimpeded as rotational velocity increases whereas increasingly coarser coal particles will pass unimpeded as rotational velocity is reduced. Conversely, for a fixed rotational velocity of the vanes, by the judicious manipulation and control of the aforesaid velocity of the primary air stream, the relative fineness of the two groups of coal particles may be adjusted, i.e., by increasing the velocity of the primary air stream, the fineness of the coal particles that pass through the aforesaid window increases. In other words finer and finer particles will pass unimpeded as primary air velocity increases and coarser and coarser coal particles will pass unimpeded as primary air velocity is reduced.
Bowl mills can be characterized by two important variables-fineness and throughput. Fineness is the cumulative percentage of the mass of a sample of particles distributed over a series of successively more restrictive standard mesh screens. According to one fineness classification method, mesh sizes range from #4 which indicates 4 openings per inch or 16 openings per square inch to #400 which indicates 400 openings per inch or 160,000 openings per square inch. A #200 mesh screen, for example, will allow particles of no greater than 74 microns to pass. Throughput is simply the mass flow rate of raw coal fed to the bowl mill. Furthermore, the operation of classifiers can be characterized by several important relationships. Firstly, the carbon loss suffered by a coal-fired steam generating power plant decays approximately exponentially with an increase in the fineness of the pulverized coal burned in the steam generator. Secondly, fineness declines approximately linearly with increasing throughput; with the performance of a rotary classifier an improvement upon that of a static classifier. Thirdly, in the pulverization of coal, for example, the log percentage of the throughput increases approximately linearly with a reduction in fineness; with the performance of a rotary classifier an improvement upon that of a static classifier. In addition, by increasing the mass flow rate of primary air to the bowl mill, fineness decreases due to the fact that heavier and therefore larger coal particles can be adequately entrained by the primary air stream. Conversely, by decreasing the mass flow rate of primary air to the mill, fineness increases. Also, by increasing the rotational velocity of a rotary classifier, fineness increases. One possible reason for the decrease in the fineness could be that there is now a smaller time interval available between the successive passage of the classifier vanes through which a particle of coal may pass. Conversely, by decreasing the rotational velocity of the rotary classifier, the fineness is decreased.
In a conventional coal-fired steam generating power plant, a multiplicity of bowl mills, of the type described above, would commonly be employed for the purpose of supplying the pulverized coal requirements thereto. As stated above, the pulverizer is an integral component of a steam generating power plant. However, the steam generating power plant is in turn an integral part of a larger electric power system which further includes a turbine/generator set and an electric power grid. In particular, pulverized coal is delivered from the pulverizers to the furnace of the steam generator wherein it is burned in air and, coupled with the working fluid of a thermodynamic steam cycle, superheated and/or reheated steam is produced thereby. The superheated and/or reheated steam is then used as the motive power to rotate a steam turbine. An electric generator, which in known fashion is cooperatively associated with the steam turbine, converts the kinetic energy of the steam turbine into electric power. This electric power is delivered to the electric power grid for consumption therein.
However, the power needs of the electric power grid are variable and may display abrupt changes. This in turn places abrupt changes in the demands placed upon the output of the furnace of a steam generating power plant, which is in turn reflected in the abrupt changes in the demands placed upon the fuel output of the pulverizers of the power plant. As an example, there may be times when the power requirements of the electric power grid increase rapidly. These power requirements are relayed to the operators of the furnace of the steam generating power plant who thereupon demand a correspondingly rapid increase in the fuel output of the pulverizers. Typical of the current methods of increasing the fuel output of the pulverizers to meet such a demand is to increase the rate at which raw, untrammeled coal is fed to the pulverizers coupled with an increase in the mass flow rate of primary air fed to the pulverizers, while allowing the rotational velocity of the rotary classifier to adjust in order to maintain a prescribed fineness profile. However, this is a relatively slow and time consuming method in which the lag time between the demand for more power received from the electric power grid and the concomitant response in an increase in fuel output by the pulverizers may be unacceptably long. Conversely, there may be times when the power requirements of the electric power grid decrease rapidly. These requirements are also relayed to the operators of the furnace of the steam generating power plant who thereupon demand a correspondingly rapid decrease in the fuel output of the pulverizers. Typical of the current methods of decreasing the fuel output of the pulverizers to meet such a demand is to decrease the rate at which raw, untrammeled coal is fed to the pulverizers coupled with a decrease in the mass flow rate of primary air fed to the pulverizers, while allowing the rotational velocity of the rotary classifier to adjust in order to maintain a prescribed fineness profile. However, this is also a relatively slow and time consuming method in which the lag time between demand for less power received from the electric power grid and the concomitant response in a decrease in the fuel output of the pulverizers may also be unacceptably long.
The present invention addresses these problems by providing a method of operating a bowl mill pulverizer such that the furnace of a steam generating power plant can more rapidly respond to abrupt changes in the demands placed upon the output of the furnace due to the abrupt changes in the power requirements of an electric power grid. In particular, the present invention accomplishes this by providing a method of delivering a slug of pulverized coal to the furnace of a steam generating power plant from either a single pulverizer or from a plurality of pulverizers operated simultaneously and in an en banc fashion or from a plurality of pulverizers operated in a sequential fashion beginning with a first bowl mill followed by a second bowl mill and so forth until each bowl mill, or as many bowl mills of the plurality as desired, has been so operated. Furthermore, the present invention accomplishes this by providing a method of withholding a slug of pulverized coal to the furnace of a steam generating power plant from either a single pulverizer or from a plurality of pulverizers operated simultaneously and in an en banc fashion or from a plurality of pulverizers operated in a sequential fashion beginning with a first bowl mill followed by a second bowl mill and so until each bowl mill, or as many bowl mills of the plurality as desired, has been so operated.
U.S. Pat. No. 5,603,268 discloses an improved method and control system for operating a coal pulverizer associated with a rotary classifier, in which current of the motor of the pulverizer can be prevented from exceeding a rated value and thus tripping of the motor can be prevented, while a high efficiency of the operation of a boiler receiving coal from the classifier is maintained. Furthermore, U.S. Pat. No. 5,386,945 discloses a roller mill control method capable of automatically controlling a roller mill which is difficult to control, and a controller for carrying out the roller mill control method. Also, U.S. Pat. No. 4,915,306 discloses a technique wherein the technique for the control of a pulverizer in a coal-fired steam generator plant is achieved by the use of a "coordination curve" which relates the primary air flow to the pulverizer with the required mass flow of coal through the pulverizer. U.S. Pat. No. 4,684,069 discloses a classifier and its controller, the classifier being operable in a vertical mill, for example, to guide a powdery material by means of a gas flow, and to selectively draw off a portion of the powdery material according to the particle size of the powdery material. Furthermore, U.S. Pat. No. 4,640,464 discloses a control system operative to control the rate of feed of material to a roller mill in accordance with the output that is being demanded from the roller mill, while yet at the same time ensuring that during changes in the output being demanded from the roller mill both a constant fineness of pulverized material and a constant air-to-solids ratio from the roller mill are being maintained. Also, U.S. Pat. No. 4,518,123 discloses a control system for a pulverizer which is capable of expanding the control range for the coal pulverizing rate, to the greatest extent possible. There is also disclosed, by way of U.S. Pat. No. 4,184,640, a roller mill with a control system wherein the control system is connected in circuit relation with prime mover means, which in turn are connected to the grinding rolls and the classifier means, respectively, of the roller mill. Moreover, U.S. Pat. No. 3,092,337 a system employing an exhauster and which includes an indicator and control system that provides an instantaneous indication of the pulverized material output and control responsive thereto. Furthermore, U.S. Pat. No. 2,831,637 discloses an improved control system for regulating the amount and temperature of air conveying the pulverized fuel from the mill to the burners into the furnace under widely varying operating and load conditions. U.S. Pat. No. 2,564,595 discloses an invention which relates to improvement in means for selectively separating finer from coarser particles of dust like material and particularly to a new and useful improvement in separating devices known as whizzer separators.
Thus, there has been evidenced in the prior art a need for a new and improved method whereby the furnace of a steam generating power plant can be made to more rapidly respond to abrupt changes in the demands placed upon the output of the furnace due to the abrupt changes in the power requirements of an electric power grid.
There has also been evidenced by the prior art a need for a new and improved method of controlling the operation of a bowl mill such that the furnace of a steam generating power plant can more rapidly respond to abrupt changes in the demands placed upon the output of the furnace due to the abrupt changes in the power requirements of an electric power grid.
It is therefore an object of the present invention to provide a new and improved method of controlling the operation of a bowl mill such that the furnace of a steam generating power plant can more rapidly respond to abrupt changes in the demands placed upon the output of the furnace due to the abrupt changes in the power requirements of an electric power grid.
Further, it is an object of the present invention to provide such a new and improved method of controlling the operation of a bowl mill such that the method is capable of simultaneous use in conjunction with each bowl mill of a plurality of bowl mills operating in an en banc fashion.
It is yet further an object of the present invention to provide such a new and improved method of controlling the operation of a bowl mill such that the method is capable of use in conjunction with each bowl mill of a plurality of bowl mills beginning with a first bowl mill and proceeding sequentially therefrom to a second bowl mill and so on until each bowl mill, or as many bowl mills as desired, of the plurality of bowl mills has been so operated.
It is still further an object of the present invention to provide such a new and improved method of controlling the operation of a bowl mill such that the method is capable of use in conjunction with numerous different types of pulverizers.